| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com> |
| Cc: | Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, Shawn Debnath <sdn(at)amazon(dot)com>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com> |
| Subject: | Re: Refactoring the checkpointer's fsync request queue |
| Date: | 2018-11-13 18:01:14 |
| Message-ID: | 20181113180114.hqporqfdnbyvlx7q@alap3.anarazel.de |
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| Lists: | pgsql-hackers |
Hi,
On 2018-11-12 15:58:41 +1300, Thomas Munro wrote:
> There is one major problem with this patch: BufferSync(), run in the
> checkpointer, can deadlock against a backend that holds a buffer lock
> and is blocked in SendFsyncRequest(). To break this deadlock, we need
> way out of it on either the sending or receiving side. Here are three
> ideas:
That's the deadlock I'd mentioned in Pune (?) btw.
> 1. Go back to the current pressure-valve strategy: make the sending
> side perform the fsync(), if it can't immediately write to the pipe.
I don't think that's correct / safe? I've previously wondered whether
there's any way we could delay the write to a point where the buffer is
not locked anymore - as far as I can tell it's actually not required for
correctness that we send the fsync request before unlocking. It's
architecturally a bit dicey tho :(
Greetings,
Andres Freund
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